As an example of a device using an elastic wave, a SAW (Surface Acoustic Wave) device is well known. The SAW device is used for various circuits (such as a transmission band-pass filter, a reception band-pass filter, a local oscillation filter, an antenna duplexer, an IF filter, and an FM modulator) that process radio signals in a frequency band of, for example, 45 MHz to 2 GHz.
FIG. 8 shows a basic structure of such an SAW device. Referring FIG. 8, the SAW device is composed of a piezoelectorc substrate 100 and IDTs (Interdigital Transducers) 101 and 102. The piezoelectric substrate 100 is composed of, for example, LiNbO.sub.3. The IDTs 101 and 102 are composed of a metal material such gas Al thin films. The IDTs 101 and 102 are formed on the piezelectric substrate 100 by etching or the like. When a racic frecuency electric signal is applied the IDT 101, a SAW 103 excited on the surface of the piezoelectric substrate 100. The excited SAW 103 is propagated to the IDT 102 through the surface of the piezoelectric substrate 100. The IDT 102 converts the SAW 103 into an electric signal.
Since the SAW device uses a boundary surface between the solid surface and vacuum or gas (namely, uses an elastic wave that is propagated through the solid surface), the surface of the piezoelectric substrate that is a propagation medium should be a free surface. Thus, chips on the SAW device cannot be packaged with plastic mold that is used for semiconductors. Thus, the SAW device should have a hollow portion that assures such a free surface.
However, when such a hollow portion is formed in the package, the cost of the device rises and the size thereof becomes larger.